Nitride-based semiconductor device and method for fabricating the same

ABSTRACT

A nitride-based semiconductor light-emitting device  100  includes: a GaN substrate  10  with an m-plane surface  12;  a semiconductor multilayer structure  20  provided on the m-plane surface  12  of the GaN substrate  10;  and an electrode  30  provided on the semiconductor multilayer structure  20.  The electrode  30  includes an Mg layer  32  and an Ag layer  34  provided on the Mg layer  32.  The Mg layer  32  is in contact with a surface of a p-type semiconductor region of the semiconductor multilayer structure  20.

TECHNICAL FIELD

The present invention relates to a nitride-based semiconductor deviceand a method for fabricating such a device. More particularly, thepresent invention relates to a GaN-based semiconductor light-emittingdevice such as a light-emitting diode or a laser diode that operates atwavelengths over the ultraviolet range and the entire visible radiationrange, which covers blue, green, orange and white parts of the spectrum.Such a light-emitting device is expected to be applied to various fieldsof technologies including display, illumination and optical informationprocessing in the near future. The present invention also relates to amethod of making an electrode for use in such a nitride-basedsemiconductor device.

BACKGROUND ART

A nitride semiconductor including nitrogen (N) as a Group V element is aprime candidate for a material to make a short-wave light-emittingdevice because its bandgap is sufficiently wide. Among other things,gallium nitride-based compound semiconductors (which will be referred toherein as “GaN-based semiconductors” and which are represented by theformula Al_(x)Ga_(y)In_(z)N (where 0≦x, y, z≦1 and x+y+z=1)) have beenresearched and developed particularly extensively. As a result, bluelight-emitting diodes (LEDs), green LEDs, and semiconductor laser diodesmade of GaN-based semiconductors have already been used in actualproducts.

A GaN-based semiconductor has a wurtzite crystal structure. FIG. 1schematically illustrates a unit cell of GaN. In an Al_(x)Ga_(y)In_(z)N(where 0≦x, y, z≦1 and x+y+z=1) semiconductor crystal, some of the Gaatoms shown in FIG. 1 may be replaced with Al and/or In atoms.

FIG. 2 shows four fundamental vectors a₁, a₂, a₃ and c, which aregenerally used to represent planes of a wurtzite crystal structure withfour indices (i.e., hexagonal indices). The fundamental vector c runs inthe [0001] direction, which is called a “c-axis”. A plane thatintersects with the c-axis at right angles is called either a “c-plane”or a “(0001) plane”. It should be noted that the “c-axis” and the“c-plane” are sometimes referred to as “C-axis” and “C-plane”.

In fabricating a semiconductor device using GaN-based semiconductors, ac-plane substrate, i.e., a substrate of which the principal surface is a(0001) plane, is used as a substrate on which GaN semiconductor crystalswill be grown. In a c-plane, however, there is a slight shift in thec-axis direction between a Ga atom layer and a nitrogen atom layer, thusproducing electrical polarization there. That is why the c-plane is alsocalled a “polar plane”. As a result of the electrical polarization, apiezoelectric field is generated in the InGaN quantum well of the activelayer in the c-axis direction. Once such a piezoelectric field has beengenerated in the active layer, some positional deviation occurs in thedistributions of electrons and holes in the active layer. Consequently,due to the quantum confinement Stark effect of carriers, the internalquantum yield decreases, thus increasing the threshold current in asemiconductor laser diode and increasing the power dissipation anddecreasing the luminous efficacy in an LED. Meanwhile, as the density ofinjected carriers increases, the piezoelectric field is screened, thusvarying the emission wavelength, too.

Thus, to overcome these problems, it has been proposed that a substrateof which the principal surface is a non-polar plane such as a (10−10)plane that is perpendicular to the [10−10] direction and that is calledan “m-plane” (m-plane GaN-based substrate) be used. As used herein,attached on the left-hand side of a Miller-Bravais index in theparentheses means a “bar” (a negative direction index). As shown in FIG.2, the m-plane is parallel to the c-axis (i.e., the fundamental vectorc) and intersects with the c-plane at right angles. On the m-plane, Gaatoms and nitrogen atoms are on the same atomic-plane. For that reason,no electrical polarization will be produced perpendicularly to them-plane. That is why if a semiconductor multilayer structure is formedperpendicularly to the m-plane, no piezoelectric field will be generatedin the active layer, thus overcoming the problems described above. The“m-plane” is a generic term that collectively refers to a family ofplanes including (10−10), (−1010), (1−100), (−1100), (01−10) and (0−110)planes.

Also, as used herein, the “X-plane growth” means epitaxial growth thatis produced perpendicularly to the X plane (where X=c or m) of ahexagonal wurtzite structure. As for the X-plane growth, the X planewill be sometimes referred to herein as a “growing plane”. A layer ofsemiconductor crystals that have been formed as a result of the X-planegrowth will be sometimes referred to herein as an “X-plane semiconductorlayer”.

CITATION LIST Patent Literature

Patent Document 1: Japanese Laid-Open Patent Publication No. 2006-24750

Patent Document 2: Japanese Patent No. 3821128

SUMMARY OF INVENTION Technical Problem

As described above, a GaN-based semiconductor device that has been grownon an m-plane substrate would achieve far more beneficial effects thanwhat has been grown on a c-plane substrate but still has the followingdrawback. Specifically, a GaN-based semiconductor device that has beengrown on an m-plane substrate has higher contact resistance of thep-electrode than what has been grown on a c-plane substrate, whichconstitutes a serious technical obstacle to using such a GaN-basedsemiconductor device that has been grown on an m-plane substrate.

Further, especially in an electrode of a light-emitting device, it isrequired to reduce the light absorption loss in an electrode section forthe purpose of improving the external quantum yield, as well as toreduce the contact resistance. A metal of a large work function whichhas been commonly used for the p-electrode of a GaN-based semiconductorlight-emitting device (Pd, Au, Pt, etc.) causes a very large lightabsorption loss. Therefore, when such a metal is used for the electrode,it is impossible to achieve a high external quantum yield. Note that“external quantum yield” refers to the ratio of the number of photonsradiated out of the light-emitting device to the number of carriersinjected into the light-emitting device.

Under the circumstances such as these, the present inventorwholeheartedly carried out extensive research to simultaneously overcomea problem that a GaN-based semiconductor device, grown on an m-plane asa non-polar plane, would have high contact resistance, and a problemthat the light absorption loss in the electrode section is high. As aresult, the inventor found an effective means for reducing the contactresistance and achieving a high external quantum yield.

The present invention was conceived in view of the above. One of themajor objects of the present invention is to provide a structure andmanufacturing method of a p-electrode, which are capable of reducing thecontact resistance of a GaN-based semiconductor device that has beenfabricated by producing a crystal growth on an m-plane substrate, andwhich are also capable of reducing the light absorption loss in theelectrode section to achieve a high external quantum yield.

Solution to Problem

The first nitride-based semiconductor device of the present inventionincludes: a nitride-based semiconductor multilayer structure including ap-type semiconductor region, a surface of the p-type semiconductorregion being an m-plane; and an electrode that is arranged on the p-typesemiconductor region, wherein the p-type semiconductor region is made ofan Al_(x)In_(y)Ga_(z)N semiconductor (where x+y+z=1, x≧0, y≧0, and z≧0),and the electrode includes an Mg layer which is in contact with thesurface of the p-type semiconductor region and an Ag layer which isprovided on the Mg layer.

In one embodiment, the Ag layer is covered with a protector electrodewhich is made of a metal different from Ag.

In one embodiment, the Ag layer is covered with a protector layer whichis made of a dielectric.

In one embodiment, the semiconductor multilayer structure includes anactive layer which includes an Al_(a)In_(b)Ga_(c)N layer (where a+b+c=1,a≧0 b≧0 and c≧0), the active layer being configured to emit light.

In one embodiment, the p-type semiconductor region is a p-type contactlayer.

In one embodiment, a thickness of the Mg layer is equal to or smallerthan a thickness of the Ag layer.

In one embodiment, in the Mg layer, a concentration of N is lower than aconcentration of Ga.

In one embodiment, the semiconductor device further includes asemiconductor substrate that supports the semiconductor multilayerstructure.

In one embodiment, the p-type semiconductor region is made of GaN.

In one embodiment, the Mg layer and the Ag layer are at least partiallymade of an alloy.

A light source of the present invention includes: a nitride-basedsemiconductor light-emitting device; and a wavelength converterincluding a phosphor that converts a wavelength of light emitted fromthe nitride-based semiconductor light-emitting device, wherein thenitride-based semiconductor light-emitting device includes anitride-based semiconductor multilayer structure including a p-typesemiconductor region, a surface of the p-type semiconductor region beingan m-plane, and an electrode that is arranged on the p-typesemiconductor region, the p-type semiconductor region is made of anAl_(x)In_(y)Ga_(z)N semiconductor (where x+y+z=1, x≧0, y≧0, and z≧0),and the electrode includes an Mg layer which is in contact with thesurface of the p-type semiconductor region and an Ag layer which isprovided on the Mg layer.

In one embodiment, the p-type semiconductor region is made of GaN.

In one embodiment, the Mg layer and the Ag layer are at least partiallymade of an alloy.

A nitride-based semiconductor device fabrication method of the presentinvention includes the steps of: (a) providing a substrate; (b) formingon the substrate a nitride-based semiconductor multilayer structureincluding a p-type semiconductor region, a surface of the p-typesemiconductor region being an m-plane; and (c) forming an electrode onthe surface of the p-type semiconductor region of the semiconductormultilayer structure, wherein step (c) includes forming an Mg layer onthe surface of the p-type semiconductor region, and forming an Ag layeron the Mg layer.

In one embodiment, step (c) further includes performing a heat treatmenton the Mg layer.

In one embodiment, the heat treatment is performed at a temperature of500° C. to 700° C.

In one embodiment, the heat treatment is performed at a temperature of550° C. to 600° C.

In one embodiment, the step of forming the Mg layer includes irradiatingMg with pulses of an electron beam such that Mg is deposited onto thesurface of the p-type semiconductor region.

In one embodiment, the method further includes removing the substrateafter step (b).

In one embodiment, the p-type semiconductor region is made of GaN.

In one embodiment, the Mg layer and the Ag layer are at least partiallyalloyed.

The second nitride-based semiconductor device of the present inventionincludes: a nitride-based semiconductor multilayer structure including ap-type semiconductor region, a surface of the p-type semiconductorregion being an m-plane; and an electrode that is arranged on the p-typesemiconductor region, wherein the p-type semiconductor region is made ofan Al_(x)In_(y)Ga_(z)N semiconductor (where x+y+z=1, x≧0, y≧0, and z≧0),and the electrode includes an Mg island provided on the surface of thep-type semiconductor region and an Ag layer provided on the Mg island.

The third nitride-based semiconductor device of the present inventionincludes: a nitride-based semiconductor multilayer structure including ap-type semiconductor region, a surface of the p-type semiconductorregion being an m-plane; and an electrode that is arranged on the p-typesemiconductor region, wherein the p-type semiconductor region is made ofan Al_(x)In_(y)Ga_(z)N semiconductor (where x+y+z=1, x≧0, y≧0, and z≧0),and the electrode includes an Mg layer which is in contact with thesurface of the p-type semiconductor region and an Ag layer which isprovided on the Mg layer, and the Mg layer is made of an Mg—Ag alloy.

The fourth nitride-based semiconductor device of the present inventionincludes: a nitride-based semiconductor multilayer structure including ap-type semiconductor region, a surface of the p-type semiconductorregion being an m-plane; and an electrode that is arranged on the p-typesemiconductor region, wherein the p-type semiconductor region is made ofan Al_(x)In_(y)Ga_(z)N semiconductor (where x+y+z=1, x≧0, y≧0, and z≧0),the electrode is composed only of an alloy layer which is in contactwith the surface of the p-type semiconductor region, and the alloy layeris made of Mg and Ag.

In one embodiment, the alloy layer is formed by forming an Mg layer soas to be in contact with the surface of the p-type semiconductor regionand an Ag layer on the Mg layer, and thereafter performing a heattreatment.

In one embodiment, the alloy layer is formed by depositing a mixture orcompound of Mg and Ag onto the surface of the p-type semiconductorregion by means of evaporation, and thereafter performing a heattreatment.

Advantageous Effects of Invention

In a nitride-based semiconductor light-emitting device according to thepresent invention, an electrode on a semiconductor multilayer structureincludes an Mg layer that is in contact with the surface (which is anm-plane) of a p-type semiconductor region. As a result, the contactresistance can be reduced. Further, an Ag layer is provided on the Mglayer to reflect light so that a high external quantum yield can beachieve.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view schematically illustrating a unit cell ofGaN.

FIG. 2 is a perspective view showing four fundamental vectors a₁, a₂, a₃and c representing a wurtzite crystal structure.

FIG. 3( a) is a schematic cross-sectional view illustrating anitride-based semiconductor light-emitting device 100 as a preferredembodiment of the present invention, and FIGS. 3( b) and 3(c) illustratethe crystal structures of an m-plane and a c-plane, respectively.

FIGS. 4( a) to 4(c) are diagrams showing the distribution of Mg and Agin the electrode.

FIG. 5A is a graph showing the current-voltage characteristic under thecondition that two Pd/Pt electrodes are in contact with a p-type GaNlayer.

FIG. 5B is a graph showing the current-voltage characteristic under thecondition that two Mg/Ag electrodes are in contact with a p-type GaNlayer.

FIG. 5C is a graph which shows the specific contact resistances (Ω·cm²)of an electrode of Pd/Pt layers and an electrode of Mg/Ag layers, eachof which is subjected to a heat treatment at an optimum temperature.

FIG. 5D is a graph illustrating the contact resistance (measured value)of a semiconductor device where a surface of the semiconductor layerwhich is in contact with the electrode (contact surface) was them-plane, and the contact resistance (measured value) of a semiconductordevice where the contact surface was the c-plane.

FIG. 5E is a graph which shows the dependence of the specific contactresistances of the electrode of Pd/Pt layers and the electrode of Mg/Aglayers on the heat treatment temperature.

FIG. 5F is a diagram showing a TLM electrode pattern.

FIG. 6 shows the photographs of the surfaces of electrodes after theheat treatments at different temperatures, which are presented assubstitutes for drawings.

FIG. 7A shows profiles of Ga atoms in the depth direction in thestructure wherein the Mg/Ag electrode is provided on the m-plane GaN,which were measured using a SIMS (Secondary Ion-microprobe MassSpectrometer).

FIG. 7B shows profiles of nitrogen atoms in the depth direction in thestructure wherein the Mg/Ag electrode is provided on the m-plane GaN,which were obtained using a SIMS.

FIG. 8 shows the graphs representing the current-voltage characteristicof light-emitting diodes which include an electrode consisting of Mg/Aglayers and the graph representing the current-voltage characteristic ofa light-emitting diode which includes a conventional electrodeconsisting of Pd/Pt layers.

FIG. 9( a) is a cross-sectional view of a nitride-based semiconductorlight-emitting device 100 according to an embodiment of the presentinvention, in which a protector electrode 50 is provided over thesurface of an electrode 30. FIG. 9( b) is a cross-sectional view of thenitride-based semiconductor light-emitting device 100 according to anembodiment of the present invention, in which a protector layer 51 isprovided over the electrode 30.

FIG. 10 is a cross-sectional view illustrating a preferred embodiment ofa white light source.

DESCRIPTION OF EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed with reference to the accompanying drawings. In the drawings,any elements shown in multiple drawings and having substantially thesame function will be identified by the same reference numeral for thesake of simplicity. It should be noted, however, that the presentinvention is in no way limited to the specific preferred embodiments tobe described below.

FIG. 3( a) schematically illustrates the cross-sectional structure of anitride-based semiconductor light-emitting device 100 as a preferredembodiment of the present invention. What is illustrated in FIG. 3( a)is a semiconductor device made of GaN semiconductors and has anitride-based semiconductor multilayer structure.

The nitride-based semiconductor light-emitting device 100 of thispreferred embodiment includes a GaN-based substrate 10, of which theprincipal surface 12 is an m-plane, a semiconductor multilayer structure20 that has been formed on the GaN-based substrate 10, and an electrode30 arranged on the semiconductor multilayer structure 20. In thispreferred embodiment, the semiconductor multilayer structure 20 is anm-plane semiconductor multilayer structure that has been formed throughan m-plane crystal growth and its principal surface is an m-plane. Itshould be noted, however, that a-plane GaN could grow on an r-planesapphire substrate in some instances. That is why according to thegrowth conditions, the principal surface of the GaN-based substrate 10does not always have to be an m-plane. In the semiconductor multilayerstructure 20 of the present invention, at least the surface of itssemiconductor region that is in contact with an electrode needs to be anm-plane.

The nitride-based semiconductor light-emitting device 100 of thispreferred embodiment includes the GaN-based substrate 10 to support thesemiconductor multilayer structure 20. However, the device 100 may haveany other substrate instead of the GaN-based substrate 10 and could alsobe used without the substrate.

FIG. 3( b) schematically illustrates the crystal structure of anitride-based semiconductor, of which the principal surface is anm-plane, as viewed on a cross section thereof that intersects with theprincipal surface of the substrate at right angles. Since Ga atoms andnitrogen atoms are present on the same atomic-plane that is parallel tothe m-plane, no electrical polarization will be produced perpendicularlyto the m-plane. That is to say, the m-plane is a non-polar plane and nopiezoelecrtric field will be produced in an active layer that growsperpendicularly to the m-plane. It should be noted that In and Al atomsthat have been added will be located at Ga sites and will replace the Gaatoms. Even if at least some of the Ga atoms are replaced with those Inor Al atoms, no electrical polarization will still be producedperpendicularly to the m-plane.

Such a GaN-based substrate, of which the principal surface is anm-plane, will be referred to herein as an “m-plane GaN-based substrate”.To obtain a nitride-based semiconductor multilayer structure that hasgrown perpendicularly to the m-plane, typically such an m-plane GaNsubstrate may be used and semiconductors may be grown on the m-plane ofthat substrate. However, the principal surface of the substrate does nothave to be an m-plane as described above, and the device as a finalproduct could already have its substrate removed.

The crystal structure of a nitride-based semiconductor, of which theprincipal surface is a c-plane, as viewed on a cross section thereofthat intersects with the principal surface of the substrate at rightangles is illustrated schematically in FIG. 3( c) just for a reference.In this case, Ga atoms and nitrogen atoms are not present on the sameatomic-plane, and therefore, electrical polarization will be producedperpendicularly to the c-plane. Such a GaN-based substrate, of which theprincipal surface is a c-plane, will be referred to herein as a “c-planeGaN-based substrate”.

A c-plane GaN-based substrate is generally used to grow GaN-basedsemiconductor crystals thereon. In such a substrate, a Ga (or In) atomlayer and a nitrogen atom layer that extend parallel to the c-plane areslightly misaligned from each other in the c-axis direction, andtherefore, electrical polarization will be produced in the c-axisdirection.

Referring to FIG. 3( a) again, on the principal surface (that is anm-plane) 12 of the m-plane GaN-based substrate 10, the semiconductormultilayer structure 20 is formed. The semiconductor multilayerstructure 20 includes an active layer 24 including anAl_(a)In_(b)Ga_(c)N layer (where a+b+c=1, a≧0, b≧0 and c≧0), and anAl_(d)Ga_(e)N layer (where d+e=1, d≧0 and e≧0) 26, which is located onthe other side of the active layer 24 opposite to the m-plane 12. Inthis embodiment, the active layer 24 is an electron injection region ofthe nitride-based semiconductor light-emitting device 100.

The semiconductor multilayer structure 20 of this preferred embodimenthas other layers, one of which is an Al_(u)Ga_(v)In_(w)N layer (whereu+v+w=1, u≧0, v≧0 and w≧0) 22 that is arranged between the active layer24 and the substrate 10. The Al_(u)Ga_(v)In_(w)N layer 22 of thispreferred embodiment has first conductivity type, which may be n-type,for example. Optionally, an undoped GaN layer could be inserted betweenthe active layer 24 and the Al_(d)Ga_(e)N layer 26.

In the Al_(d)Ga_(e)N layer 26, the mole fraction d of Al does not haveto be uniform, but could vary either continuously or stepwise, in thethickness direction. In other words, the Al_(d)Ga_(e)N layer 26 couldhave a multilayer structure in which a number of layers with mutuallydifferent Al mole fractions d are stacked one upon the other, or couldhave its dopant concentration varied in the thickness direction. Toreduce the contact resistance, the uppermost portion of theAl_(d)Ga_(e)N layer 26 (i.e., the upper surface region of thesemiconductor multilayer structure 20) is preferably a layer that has anAl mole fraction d of zero (i.e., a GaN layer).

The electrode 30 is provided on the semiconductor multilayer structure20. The electrode 30 of the present embodiment includes an Mg layer 32and an Ag layer 34 formed on the Mg layer 32. The Mg layer 32 and the Aglayer 34 may be at least partially made of an alloy. Specifically, onlya boundary part of the Mg layer 32 and the Ag layer 34 may be made of analloy. Alternatively, the entire electrode 30 may be made of an alloy.

FIGS. 4( a) to 4(c) are diagrams for illustrating the process ofalloying the Mg layer 32 and the Ag layer 34. FIG. 4( a) shows a stateof the structure in which the Mg layer 32 and the Ag layer 34 have beenpartially alloyed. In this case, as shown in FIG. 4( a), the electrode30A includes the Mg layer 32 that is in contact with the Al_(d)Ga_(e)Nlayer 26, an Mg—Ag alloy layer 61A lying over the Mg layer 32, and theAg layer 34 lying over the Mg—Ag alloy layer 61A.

FIG. 4( b) shows a state of the structure in which alloying of Mg and Aghas advanced such that the alloyed portion is in contact with theAl_(d)Ga_(e)N layer 26. In the state shown in FIG. 4( b), the Mg layer32 included in the electrode 30B (a portion of the electrode 30B whichis in contact with the Al_(d)Ga_(e)N layer 26) is made of an Mg—Agalloy.

In the example of the electrode 30B shown in FIG. 4( b), the Ag layer 34is lying over the Mg layer 32.

FIG. 4( c) shows a state of the electrode 30C in which the Mg layer andthe Ag layer have been entirely alloyed. In this state, the electrode30C is composed only of an Mg—Ag alloy layer 61C.

The Mg—Ag alloys shown in FIGS. 4( a) to 4(c) are made of Mg and Ag(i.e., the major constituents are Mg and Ag). The structures shown inFIGS. 4( a) to 4(c) can be formed by forming an Ag layer on an Mg layerand thereafter performing a heat treatment on these layers. Note thatthe structure shown in FIG. 4( c) may be formed by performing a vapordeposition using a mixture or compound of Mg and Ag as a source materialand thereafter performing a heat treatment on the deposited material.

The Ag layer 34 may be an alloy layer made of Ag as a major constituentand one or more metal additives different from Ag (for example, Cu, Au,Pd, Nd, Sm, Sn, In, Bi) of a very small amount. The Ag layer 34 formedby alloying such metals is superior to Ag in terms of, for example, heatresistance and reliability.

The Ag layer has a high reflectance for light. For example, comparing interms of the reflectance for blue light, Ag reflects about 97%, Ptreflects about 55%, and Au reflects about 40%.

The Mg layer 32 included in the electrode 30 is in contact with a p-typesemiconductor region of the semiconductor multilayer structure 20 andfunctions as part of the p-electrode. In the present embodiment, the Mglayer 32 is in contact with the Al_(d)Ga_(e)N layer 26 that is dopedwith a dopant of the second conductivity type (p-type). TheAl_(d)Ga_(e)N layer 26 may be doped with, for example, Mg as a dopant.Alternatively, the Al_(d)Ga_(e)N layer 26 may be doped with any otherp-type dopant than Mg, for example, Zn or Be.

Note that at least part of the Mg layer 32 may undergo aggregation toform islands over the surface of the Al_(d)Ga_(e)N layer 26 due to theheat treatment performed after the formation of the layer, so that theislands are separated from one another with spaces. In this case, Agatoms that constitute the Ag layer 34 intervene between the respectiveMg islands. At least part of the Ag layer 34 may undergo aggregation toform islands.

In the present embodiment, the thickness of the electrode 30 is forexample from 10 nm to 200 nm. In the electrode 30, the thickness of theMg layer 32 is smaller than that of the Ag layer 34. The preferredthickness of the Mg layer 32 is, for example, from 0.5 nm to 10 nm. Notethat “the thickness of the Mg layer 32” herein refers to the thicknessof the Mg layer after the heat treatment. The reason why the preferredthickness of the Mg layer 32 is 10 nm or less is to give the Mg layer 32a light transmitting property. If the thickness of the Mg layer 32 is 10nm or less, light radiated from the active layer 24 of the semiconductormultilayer structure 20 is scarcely absorbed by the Mg layer 32 to reachthe Ag layer 34. Therefore, larger part of the light is reflected by theAg layer 34. The thickness of the Mg layer 32 is preferably smaller. Forexample, it is preferably from 1 nm to 2 nm. When the reflection oflight by the Ag layer 34 is not expected, the thickness of the Mg layer32 may not necessarily be 10 nm or less. When the thickness of the Mglayer 32 is 45 nm or greater, the contact resistance is approximatelyequal to that obtained in the case where a conventional Pd-basedelectrode is used. Further, a problem of peeling off of the electrodemay occur. Thus, the thickness of the Mg layer 32 is preferably 45 nm orless.

The thickness of the Ag layer 34 is, for example, from 10 nm to 200 nm.Since the penetration depth of light (e.g., light at a blue rangewavelength) in the Ag layer 34 is about 10 nm, the Ag layer 34 cansufficiently reflects the light so long as the thickness of the Ag layer34 is not less than the penetration depth, 10 nm. The reason why thethickness of the Mg layer 32 is smaller than that of the Ag layer 34 isto prevent separation of the Mg layer 32 and the Al_(d)Ga_(e)N layer 26which would be caused due to disturbed balance of strain between the Mglayer 32 and the Ag layer 34.

Meanwhile, the GaN-based substrate 10, of which the principal surface 12is an m-plane, may have a thickness of 100 μm to 400 μm, for example.This is because if the wafer has a thickness of at least approximately100 μm, then there will be no trouble handling such a wafer. It shouldbe noted that as long as the substrate 10 of this preferred embodimenthas an m-plane principal surface 12 made of a GaN-based material, thesubstrate 10 could have a multilayer structure. That is to say, theGaN-based substrate 10 of this preferred embodiment could also refer toa substrate, at least the principal surface 12 of which is an m-plane.That is why the entire substrate could be made of a GaN-based material.Or the substrate may also be made of the GaN-based material and anothermaterial in any combination.

In the structure of this preferred embodiment, an electrode 40 has beenformed as an n-side electrode on a portion of an n-typeAl_(u)Ga_(v)In_(w)N layer 22 (with a thickness of 0.2 μm to 2 μm, forexample) which is located on the substrate 10. In the exampleillustrated in FIG. 3( a), in the region of the semiconductor multilayerstructure 20 where the electrode 40 is arranged, a recess 42 has beencut so as to expose a portion of the n-type Al_(u)Ga_(v)In_(w)N layer22. And the electrode has been formed on the exposed surface of then-type Al_(u)Ga_(v)In_(w)N layer 22 at the bottom of the recess 42. Theelectrode 40 may have a multilayer structure consisting of Ti, Al and Tilayers and may have a thickness of 100 nm to 200 nm, for example.

In this preferred embodiment, the active layer 24 has a GaInN/GaNmulti-quantum well (MQW) structure (with a thickness of 81 nm, forexample) in which Ga_(0.9)In_(0.1)N well layers (each having a thicknessof 9 nm, for example) and GaN barrier layers (each having a thickness of9 nm, for example) are alternately stacked one upon the other.

On the active layer 24, stacked is the p-type Al_(d)Ga_(e)N layer 26,which may have a thickness of 0.2 μm to 2 μm. Optionally, an undoped GaNlayer could be inserted between the active layer 24 and theAl_(d)Ga_(e)N layer 26 as described above.

In addition, a GaN layer of the second conductivity type (which may bep-type, for example) could be formed on the Al_(d)Ga_(e)N layer 26.Furthermore, a contact layer of p⁺-GaN and the Mg layer 32 could bestacked in this order on that GaN layer. In that case, the GaN contactlayer could also be regarded as forming part of the Al_(d)Ga_(e)N layer26, not a layer that has been stacked separately from the Al_(d)Ga_(e)Nlayer 26.

Next, the feature and specificity of the present embodiment aredescribed in more detail with reference to FIG. 5A to FIG. 6.

FIG. 5A shows the current-voltage characteristic under the conditionthat two Pd/Pt electrodes are in contact with a p-type GaN layer. FIG.5B shows the current-voltage characteristic under the condition that twoMg/Ag electrodes are in contact with a p-type GaN layer. The Pd/Ptelectrode used herein was an electrode formed by sequentially forming aPd layer and a Pt layer in this order on a p-type m-plane GaN layer andthereafter performing a heat treatment on the resultant structure(m-plane GaN (Pd/Pt) electrode). The Mg/Ag electrode used herein was anelectrode formed by sequentially depositing an Mg layer and an Ag layerin this order by means of evaporation on a p-type m-plane GaN layer andthereafter performing a heat treatment on the resultant structure(m-plane GaN (Mg/Ag) electrode). The Mg layer included in the Mg/Agelectrode was deposited using a pulse evaporation process. The pulseevaporation process will be described later. In any of the experimentalexamples described in the present specification, the Mg layer wasdeposited using the pulse evaporation process, and the Ag layer wasdeposited by a common electron beam evaporation process.

The structures and heat treatment conditions of the Pd/Pt electrode andthe Mg/Ag electrode are shown below in TABLE 1.

TABLE 1 Heat treatment Plane Thickness (before temperature andorientation p-electrode heat treatment) duration m-plane Pd/Pt 40 nm/35nm 500° C., 10 min. m-plane Mg/Ag  7 nm/75 nm 600° C., 10 min.

The Mg/Ag electrodes and the Pd/Pt electrodes are in contact with them-plane GaN layer doped with Mg. In the m-plane GaN layer that is incontact with these electrodes, a region of the layer extending from thelayer surface to the depth of 20 nm (uppermost surface region of 20 nmthick) is doped with Mg at 7×10¹⁹ cm⁻³. The remaining part of them-plane GaN layer, deeper than 20 nm from the layer surface, is dopedwith Mg at 1×10¹⁹ cm⁻³. If the concentration of the p-type impurity islocally increased in this manner in the uppermost surface region of theGaN layer that is in contact with the p-electrode, the contactresistance can be reduced to the lowest possible level. By performingsuch an impurity doping, the in-plane non-uniformity of thecurrent-voltage characteristic can also be reduced. As a result, thevariation in drive voltage among chips can also be advantageouslyreduced. That is why in every experimental example disclosed in thisapplication, the surface region of the p-type GaN layer that is incontact with the electrode, extending from the layer surface to thedepth of 20 nm, is doped with Mg at 7×10¹⁹ cm⁻³, while the deeper regionis doped with Mg at 1×10¹⁹ cm⁻³.

The curves of the current-voltage characteristic shown in FIGS. 5A and5B respectively correspond to the distances between electrodes of theTLM (Transmission Line Method) electrode pattern shown in FIG. 5F. FIG.5F shows an arrangement of a plurality of electrodes of 100 μm×200 μmwith the intervals of 8 μm, 12 μm, 16 μm, and 20 μm.

Pd is a metal of a large work function, which has been conventionallyused for the p-electrode. In the Pd/Pt electrode, the Pd layer is incontact with the p-type GaN layer. The graph of FIG. 5A (thecurrent-voltage characteristic of the Pd/Pt electrode) shows aSchottky-type non-ohmic characteristic (Schottky voltage: about 2 V). Onthe other hand, no Schottky voltage is seen in the graph of FIG. 5B (thecurrent-voltage characteristic of the Mg/Ag electrode). Thus, it can beunderstood that this Mg/Ag electrode substantially forms an ohmiccontact with the p-type GaN layer. Disappearance of the Schottky voltageis critical in decreasing the operating voltages of devices, such aslight-emitting diodes, laser diodes, etc.

FIG. 5C is a graph showing the specific contact resistances (Ω·cm²) ofthe Pd/Pt electrode and the Mg/Ag electrode that have been describedabove. The Pd/Pt electrode was subjected to a heat treatment at 500° C.,and the Mg/Ag electrode was subjected to a heat treatment at 600° C. Thestructures and heat treatment conditions of the Pd/Pt electrode and theMg/Ag electrode that were measured in terms of the specific contactresistance as shown in FIG. 5C are the same as those of TABLE 1.

The contact resistance was evaluated using the TLM. Referring to theordinate axis, “1.0E-01” means “1.0×10⁻¹”, and “1.0E-02” means“1.0×10⁻²”. Hence, “1.0E+X” means “1.0×10^(X)”.

In general, contact resistance R is inversely proportional to the areaof the contact, S (cm²). Where R(Ω) is the contact resistance, therelationship of R=Rc/S holds. The proportionality constant, Rc, iscalled specific contact resistance, which equals to the contactresistance R when the contact area S is 1 cm². Thus, the value of thespecific contact resistance does not depend on the contact area S andhence serves as an index for evaluation of the contact characteristic.Hereinafter, “specific contact resistance” is sometimes abbreviated as“contact resistance”.

As shown in FIG. 5C, the Mg/Ag electrode exhibits a lower specificcontact resistance (Ω·cm²) than the Pd/Pt electrode by approximately oneorder of magnitude.

FIG. 5D is a graph illustrating the contact resistance (measured value)of a semiconductor device where a surface of the semiconductor layerwhich is in contact with the electrode (contact surface) was them-plane, and the contact resistance (measured value) of a semiconductordevice where the contact surface was the c-plane. In the samples used inthis measurement, any of the Mg/Ag electrode and the Pd/Pt electrode wasin contact with the p-type GaN layer. In any of the samples, a region ofthe p-type GaN layer that was in contact with the electrode, extendingfrom the layer surface to the depth of 20 nm, was doped with Mg at7×10¹⁹ cm⁻³, and the deeper region was doped with Mg at 1×10¹⁹ cm⁻³.

As apparent from FIG. 5D, when the contact surface is the c-plane, theMg/Ag electrode exhibits a slightly lower contact resistance than thePd/Pt electrode. However, when the contact surface is the m-plane, thecontact resistance of the Mg/Ag electrode is significantly lower thanthat of the Pd/Pt electrode.

Next, the dependence of the contact resistance on the heat treatmenttemperature is described. The conventional Pd/Pt electrode and the Mg/Agelectrode of the present embodiment are separately described. FIG. 5E isa graph which shows the dependence of the specific contact resistancevalues of the Pd/Pt electrode and the Mg/Ag electrode on the heattreatment temperature.

In the Mg/Ag electrode before the heat treatment, the thickness of theMg layer was 7 nm, and the thickness of the Ag layer was 75 nm. In thePd/Pt electrode before the heat treatment, the thickness of the Pd layerwas 40 nm, the thickness of the Pt layer was 35 nm.

As seen from FIG. 5E, in the case of the m-plane GaN (Pd/Pt) electrode,the contact resistance of the m-plane GaN scarcely changed after theheat treatment at 500° C. At heat treatment temperatures higher than500° C., an increase of the contact resistance was detected.

On the other hand, in the case of the m-plane GaN (Mg/Ag) electrode, thecontact resistance sharply decreased at temperatures higher than 500° C.And, at 600° C., the contact resistance further decreased. When thetemperature was further increased to 700° C., the contact resistance washigher than that obtained when the heat treatment temperature was 600°C. but was smaller than the contact resistance obtained in the case ofthe conventional m-plane GaN (Pd/Pt) electrode.

Therefore, the heat treatment temperature for the m-plane GaN (Mg/Ag)electrode is preferably 500° C. or higher, for example. The upper limitof the heat treatment temperature is preferably 700° C. or less because,if it exceeds 700° C. to reach a predetermined temperature (e.g., 800°C.) or higher, deterioration in the film quality of the electrode andthe GaN layer would increase.

Next, photographs of the surfaces of electrodes after the heattreatments at different temperatures are shown in FIG. 6. FIG. 6 showsthe photographs of as-deposited electrodes (without heat treatment) andelectrodes subjected to heat treatments at 500° C., 600° C., and 700° C.

As seen from FIG. 6, in the electrodes where the Pd layer was providedon the p-type m-plane GaN layer and the Pt layer was provided on the Pdlayer (m-plane GaN (Pd/Pt) electrode), roughened metal surfaces weredetected, i.e., degradation of the metal surfaces was recognized, afterthe heat treatments at 600° C. and 700° C. Note that it was foundthrough the experiments of the present inventor that no roughened metalsurface was detected in a Pd/Pt electrode which was formed on a c-planeGaN layer and then subjected to a heat treatment at a temperature from600° C. to 700° C. In can be appreciated from these results thatdegradation of an electrode due to the heat treatment is a problemspecific to an electrode formed on m-plane GaN.

On the other hand, in the case where the Mg layer was provided on thep-type m-plane GaN layer and the Ag layer was provided on the Mg layer(the m-plane GaN (Mg/Ag) electrode of the present embodiment), onlysmall surface irregularities were detected after the heat treatment at700° C. However, it was confirmed that no significant degradation of theelectrodes was detected at any of 500° C., 600° C., and 700° C.

It is understood from the measurement results of the contact resistanceshown in FIG. 5E that, in the case of the m-plane GaN (Mg/Ag) electrode,the contact resistance has the lowest value when the heat treatmenttemperature is around 600° C. (e.g., 600° C.±50° C.). On the other hand,it is also understood from the results shown in FIG. 6 that, in them-plane GaN (Mg/Ag) electrode, the surface degradation is small evenwhen the heat treatment temperature is as high as 700° C., but the stateof the surface of the electrode is maintained better as the heattreatment temperature is lower. Degradation of the surface of the Aglayer would lead to a decrease in reflectance, and hence, the surface ofthe electrode is preferably maintained to have a desirable surfacecondition. In view of the balance between the contact resistance valueand the electrode surface condition, it can be considered that using aheat treatment temperature from 550° C. to 600° C. is especiallypreferred.

Generally, in fabrication of an excellent p-electrode of a low contactresistance, using a metal of a large work function, for example, Pd(work function=5.1 eV) or Pt (work function=5.6 eV), is common knowledgein the art. The present inventor used a variety of metals of differentwork functions, such as Al, Ni, Au, Pd, Pt, etc., as the material of theelectrodes for m-plane GaN, and measured the contact resistance of theelectrodes. As a result, the present inventor experimentallydemonstrated that, even in the case of the m-plane GaN, lower contactresistances are achieved by metals of larger work functions (Pd and Pt),and confirmed that a p-electrode of low contact resistance would not beobtained even when such a metal of large work function is used. On theother hand, the work function of Mg is only 3.7 eV, and therefore, theidea of using Mg for the p-electrode is unlikely to be arrived at.

However, upon evaluation of the contact resistance of the Mg/Agelectrode of the present embodiment, the Mg/Ag electrode successfullyexhibited a lower specific contact resistance than theconventionally-employed Pd/Pt electrode of a large work function byapproximately one order of magnitude, which is an exceptionallyadvantageous effect of the present embodiment.

The reason why the contact resistance greatly decreases when theelectrode structure of the present embodiment (Mg/Ag) is provided on them-plane GaN is inferred to be that the heat treatment allows only Gaatoms of the GaN to be diffused toward the electrode side while N atomsare not diffused toward the electrode side. It is inferred that only Gaof the GaN is diffused toward the electrode side, and accordingly, theconcentration of N is lower than the concentration of Ga in the Mglayer.

When Ga atoms of the p-type GaN are diffused toward the electrode side,the outermost surface of the p-type GaN is lacking Ga atoms, i.e., Gavacancies are formed. The Ga vacancies have acceptor-like properties,and therefore, as the number of Ga vacancies increases in the vicinityof the interface between the electrode and the p-type GaN, holes morereadily pass through the Schottky barrier of this interface by means oftunneling. Thus, it is inferred that the contact resistance decreaseswhen the Mg layer is formed so as to be in contact with the m-planesurface of the p-type GaN layer.

On the other hand, when N atoms as well as Ga atoms are diffused towardthe electrode side, the outermost surface of the p-type GaN is lacking Natoms, i.e., N vacancies are also formed. Since the N vacancies havedonor-like properties, charge compensation occurs between the Gavacancies and the N vacancies at the outermost surface of the p-typeGaN. It is also inferred that the omission of the N atoms would degradethe crystallinity of GaN crystals. Thus, when N atoms as well as Gaatoms are diffused toward the electrode side, the contact resistancebetween the p-type GaN layer and the electrode is high.

This finding teaches that the physical properties, such as interatomicbond strength, surface condition, etc., are totally different betweenthe m-plane GaN and the c-plane GaN.

FIG. 7A shows profiles of Ga atoms in the depth direction in thestructure wherein the Mg/Ag electrode was provided on the m-plane GaN,which were measured using a SIMS. FIG. 7A shows the profile obtainedbefore the heat treatment (as-depo) and the profile obtained after theheat treatment (after 600° C. heat treatment). The ordinate axis of thegraph represents the intensity (corresponding to the Ga concentration),and the abscissa axis represents the distance in the depth direction.The intensity of 1×10¹ substantially corresponds to the Ga concentrationof 1×10¹⁹ cm⁻³. In the abscissa axis, the negative (−) value range is onthe electrode side, and the positive (+) value range is on the p-typeGaN side. The origin of the abscissa axis (0 μm) is coincident with apeak position of Mg and approximately corresponds to the position of theinterface between the p-type GaN layer and the Mg layer.

The heat treatment on the samples used for the measurement was performedat 600° C. for 10 minutes. Before the heat treatment, the thickness ofthe Mg layer was 7 nm, and the thickness of the Ag layer was 75 nm.Before the heat treatment, in any of the samples, a region of the p-typeGaN layer that was in contact with the electrode, extending from thelayer surface to the depth of 20 nm, was doped with Mg at 7×10¹⁹ cm⁻³,while the deeper region was doped with Mg at 1×10¹⁹ cm⁻³.

As shown in FIG. 7A, in the “as-depo” state, the Ga concentrationmonotonically decreases as the abscissa value becomes closer to theelectrode surface side (negative (−) side), and goes beyond thedetection limit in the vicinity of −0.02 μm. It is understood from thisresult that, in the “as-depo” state, Ga was scarcely diffused to themetal side. On the other hand, after the heat treatment, a plateauregion was detected in a region ranging from −0.006 μm to −0.04 μm. Itis understood from this result that, as compared with the data obtainedbefore the heat treatment, Ga was diffused to the electrode surfaceside.

Next, the behavior of nitrogen is described. FIG. 7B shows profiles ofnitrogen atoms in the depth direction in the structure wherein the Mg/Agelectrode was provided on the m-plane GaN, which were obtained using aSIMS. In the ordinate axis, the intensity of 1×10¹ substantiallycorresponds to the N concentration of 1×10¹⁹ cm⁻³. The ordinate axis ofthe graph represents the intensity (corresponding to the Nconcentration), and the abscissa axis represents the distance in thedepth direction. In the abscissa axis, the negative (−) value range ison the electrode side, and the positive (+) value range is on the p-typeGaN side. The origin of the abscissa axis (0 μm) is coincident with apeak position of Mg and approximately corresponds to the position of theinterface between the p-type GaN layer and the Mg layer.

The heat treatment on the samples used for the measurement was performedat 600° C. for 10 minutes. Before the heat treatment, the thickness ofthe Mg layer was 7 nm, and the thickness of the Ag layer was 75 nm. Thestructure of the electrode and the doping conditions for the p-type GaNwere the same as those of the samples from which the measurement resultsof FIG. 7A were obtained.

As shown in FIG. 7B, the profile of nitrogen in the depth direction didnot largely change after the heat treatment. It is understood from thisresult that the nitrogen atoms were not greatly diffused toward theelectrode side.

It is understood from the results shown in FIGS. 7A and 7B that, in them-plane GaN, only the Ga atoms were diffused to the electrode side,while the nitrogen atoms were not diffused.

It is inferred that the behaviors of respective ones of such elements(Ga, N) occur even when some of Ga atoms are replaced by Al or In atomsin the GaN layer with which the Mg layer is in contact. It is alsoinferred that the same applies even when the GaN-based semiconductorlayer with which the Mg layer is in contact is doped with an elementother than Mg as a dopant.

Next, referring again to FIG. 3( a), the structure of the presentembodiment is described in more detail.

As shown in FIG. 3( a), the light-emitting device 100 of the presentembodiment includes the m-plane GaN substrate 10 and theAl_(u)Ga_(v)In_(w)N layer 22 (where u+v+w=1, u≧0, v≧0, w≧0) provided onthe substrate 10. In this example, the m-plane GaN substrate 10 is ann-type GaN substrate (for example, 100 μm thick). TheAl_(u)Ga_(v)In_(w)N layer 22 is an n-type GaN layer (for example, 2 μmthick). Provided on the Al_(u)Ga_(v)In_(w)N layer 22 is an active layer24. In other words, a semiconductor multilayer structure 20 including atleast the active layer 24 is provided on the m-plane GaN substrate 10.

In the semiconductor multilayer structure 20, an active layer 24including an Al_(a)In_(b)Ga_(c)N layer (where a+b+c=1, a≧0, b≧0 and c≧0)has been formed on the Al_(x)Ga_(y)In_(z)N layer 22. The active layer 24consists of InGaN well layers with an In mole fraction of approximately25% and GaN barrier layers, both the well layers and the barrier layersmay have a thickness of 9 nm each, and the well layers may have a welllayer period of three. On the active layer 24, stacked is anAl_(d)Ga_(e)N layer (where d+e=1, d≧0 and e≧0) 26 of the secondconductivity type (which may be p-type, for example), which may be anAlGaN layer with an Al mole fraction of 10% and may have a thickness of0.2 μm. In this preferred embodiment, the Al_(d)Ga_(e)N layer 26 isdoped with Mg as a p-type dopant to a level of approximately 10¹⁸ cm⁻³,for example. Also, in this example, an undoped GaN layer (not shown) isinterposed between the active layer 24 and the Al_(d)Ga_(e)N layer 26.

Furthermore, in this example, on the Al_(d)Ga_(e)N layer 26, stacked isa GaN layer (not shown) of the second conductivity type (which may bep-type, for example). In addition, on the contact layer of p⁺-GaN,stacked in this order are an Mg layer 32 and an Ag layer 34. And thisstack of the Mg layer 32 and the Ag layer 34 is used as an electrode(i.e., a p-electrode) 30.

This semiconductor multilayer structure 20 further has a recess 42 thatexposes the surface of the Al_(u)Ga_(v)In_(w)N layer 22. And anelectrode 40 (n-electrode) has been formed on the Al_(u)Ga_(v)In_(w)Nlayer 22 at the bottom of the recess 42, which may have a width (ordiameter) of 20 μm and a depth of 1 μm, for example. The electrode 40may have a multilayer structure consisting of Ti, Al and Pt layers,which may have thicknesses of 5 nm, 100 nm and 10 nm, respectively.

The present inventors discovered that the nitride-based semiconductorlight-emitting device 100 of this preferred embodiment could have anoperating voltage Vop that was approximately 2.0 V lower than that of aconventional m-plane LED with a Pd/Pt electrode, and therefore, couldcut down the power dissipation as a result.

Further, it was confirmed that the external quantum yield greatlyimproves due to the light reflecting effect of the Ag layer 34 ascompared with the m-plane LED which includes the conventional Pd/Ptelectrode.

Next, a method for fabricating the nitride-based semiconductorlight-emitting device 100 of this embodiment is described while stillreferring to FIG. 3( a).

First of all, an m-plane substrate 10 is prepared. In this preferredembodiment, a GaN substrate is used as the substrate 10. The GaNsubstrate of this preferred embodiment is obtained by HVPE (hydridevapor phase epitaxy).

For example, a thick GaN film is grown to a thickness of severalmillimeters on a c-plane sapphire substrate, and then dicedperpendicularly to the c-plane (i.e., parallel to the m-plane), therebyobtaining m-plane GaN substrates. However, the GaN substrate does nothave to be prepared by this particular method. Alternatively, an ingotof bulk GaN may be made by a liquid phase growth process such as asodium flux process or a melt-growth method such as an ammonothermalprocess and then diced parallel to the m-plane.

The substrate 10 does not have to be a GaN substrate but may also be agallium oxide substrate, an SiC substrate, an Si substrate or a sapphiresubstrate, for example. To grow an m-plane GaN-based semiconductor onthe substrate by epitaxy, the principal surface of the SiC or sapphiresubstrate is preferably also an m-plane. However, in some instances,a-plane GaN could grow on an r-plane sapphire substrate. That is whyaccording to the growth conditions, the surface on which the crystalgrowth should take place does not always have to be an m-plane. In anycase, at least the surface of the semiconductor multilayer structure 20should be an m-plane. In this preferred embodiment, crystal layers areformed one after another on the substrate 10 by MOCVD (metalorganicchemical vapor deposition) process.

Next, an Al_(u)Ga_(v)In_(w)N layer 22 is formed on the m-plane GaNsubstrate 10. As the Al_(u)Ga_(v)In_(w)N layer 22, AlGaN may bedeposited to a thickness of 3 μm, for example. A GaN layer may bedeposited by supplying TMG(Ga(CH₃)₃), TMA(Al(CH₃)₃) and NH₃ gases ontothe m-plane GaN substrate 10 at 1,100° C., for example.

Subsequently, an active layer 24 is formed on the Al_(u)Ga_(v)In_(w)Nlayer 22. In this example, the active layer 24 has a GaInN/GaNmulti-quantum well (MQW) structure in which Ga_(0.9)In_(0.1)N welllayers and GaN barrier layers, each having a thickness of 9 nm, havebeen stacked alternately to have an overall thickness of 81 nm. When theGa_(0.9)In_(0.1)N well layers are formed, the growth temperature ispreferably lowered to 800° C. to introduce In.

Thereafter, an undoped GaN layer is deposited to a thickness of 30 nm,for example, on the active layer 24, and then an Al_(d)Ga_(e)N layer 26is formed on the undoped GaN layer. As the Al_(d)Ga_(e)N layer 26,p-Al_(0.14)Ga_(0.86)N is deposited to a thickness of 70 nm by supplyingTMG, NH₃, TMA, TMI gases and Cp₂Mg (cyclopentadienyl magnesium) gas as ap-type dopant.

Next, a p-GaN contact layer is deposited to a thickness of 0.5 μm, forexample, on the Al_(d)Ga_(e)N layer 26. In forming the p-GaN contactlayer, Cp₂Mg is supplied as a p-type dopant.

Thereafter, respective portions of the p-GaN contact layer, theAl_(d)Ga_(e)N layer 26, the undoped GaN layer, and the active layer 24are removed by performing a chlorine-based dry etching process, therebymaking a recess 42 and exposing a region of the Al_(x)Ga_(y)In_(z)Nlayer 22 where an n-electrode will be formed. Then, Ti/Al/Pt layers aredeposited as an n-electrode 40 on the region reserved for an n-typeelectrode at the bottom of the recess 42.

Then, an Mg layer 32 is formed on the p-GaN contact layer. On the Mglayer 32, an Ag layer 34 is formed using a common vapor depositionmethod (a resistance heating method, an electron beam evaporationprocess, or the like). In this way, a p-type electrode 30 is formed. Thepresent embodiment uses, for formation of the Mg layer 32, a method inwhich deposition is performed while a material metal is evaporated inpulses. More specifically, metal Mg contained in a crucible in a vacuumis irradiated with pulses of electron beam, whereby the material metalis evaporated in pulses. Molecules or atoms of that material metal aredeposited on the p-GaN contact layer, whereby the Mg layer 32 is formed.For example, those pulses may have a pulse width of 0.5 seconds and maybe applied repeatedly at a frequency of 1 Hz. By adopting such a method,a dense film of high quality could be formed as the Mg layer 32. The Mglayer had such high density probably because, by performing such a pulseevaporation, Mg atoms or a cluster of Mg atoms that collide against thep-GaN contact layer would have their kinetic energy increased. Generallyspeaking, Mg is an element which is susceptible to oxidation whenexposed to water or air. However, an Mg layer obtained by using thepulse evaporation process of the present embodiment is resistant tooxidation and exhibits excellent water resistance and oxygen resistance.Also, it was confirmed that the Mg layer formed in such a way was stableeven after the heat treatment at a temperature not less than 600° C. Thepresent embodiment uses such a method that the deposition progresseswhile the material metal (metal Mg) is evaporated in pulses. However,any other method can also be adopted so long as the Mg layer 32 can beformed (especially, so long as the formed Mg layer 32 is dense and ofhigh quality). As an alternative method, for example, sputtering, athermal CVD process, or a molecular beam epitaxy (MBE) could also beused.

Optionally, the substrate 10 and a portion of the Al_(u)Ga_(v)In_(w)Nlayer 22 could be removed after that by some technique such as laserlift-off, etching or polishing. In that case, either only the substrate10 or the substrate 10 and a portion of the Al_(u)Ga_(v)In_(w)N layer 22could be removed selectively. It is naturally possible to leave thesubstrate 10 and the Al_(u)Ga_(v)In_(w)N layer 22 as they are withoutremoving them. By performing these process steps, the nitride-basedsemiconductor light-emitting device 100 of this preferred embodiment iscompleted.

In the nitride-based semiconductor light-emitting device 100 of thispreferred embodiment, when a voltage is applied to between the n- andp-electrodes 40 and 30, holes are injected from the p-electrode 30 intothe active layer 24 and electrons are injected from the n-electrode 40into the active layer 24, thus producing photoluminescence with awavelength of about 450 nm.

FIG. 8 shows the current-voltage characteristic of a light-emittingdiode which includes an electrode consisting of Mg/Ag layers (a samplesubjected to a heat treatment at 575° C. for 10 minutes and a samplesubjected to a heat treatment at 600° C. for 10 minutes). For comparisonpurposes, the characteristic of a light-emitting diode which has thesame nitride-based semiconductor structure of the light-emitting diodebut includes an electrode consisting of Pd/Pt layers is also showntogether. In the Mg/Ag electrode before the heat treatment, thethickness of the Mg layer was 7 nm, and the thickness of the Ag layerwas 75 nm. In the Pd/Pt electrode before the heat treatment, thethickness of the Pd layer was 40 nm, and the thickness of the Pt layerwas 35 nm.

In each of these light-emitting diodes, an n-type GaN layer, an activelayer in which three InGaN well layers and two GaN barrier layers arealternately stacked one upon the other, and a p-type GaN layer arestacked in this order on an m-plane GaN substrate. In addition, eitheran Mg/Ag electrode or a Pd/Pt electrode is provided as a p-electrode onthe p-type GaN layer. On the other hand, an n-electrode is formed on then-type GaN layer by etching the p-type GaN layer and the active layerand exposing the n-type GaN layer.

The threshold voltage of the light-emitting diode which includes theelectrode of Pd/Pt layers was about 3.7 V, whereas the threshold voltageof the light-emitting diode which includes the electrode of Mg/Ag layerswas about 2.7 V. This means a considerable reduction of the thresholdvoltage. On the other hand, comparing in terms of the operating voltagefor the current value of 20 mA, it is seen that the operating voltage ofthe light-emitting diode which includes the electrode of Mg/Ag layers issmaller than that of the light-emitting diode which includes theelectrode of Pd/Pt layers by 2.0 V or more.

In the present embodiment, as shown in FIG. 9( a), the surface of theelectrode 30 consisting of the Mg layer 32 and the Ag layer 34 may becovered with a protector electrode 50 made of a metal other than Ag (forexample, Ti, Pt, Mo, Pd, Au, W, or the like). However, the lightabsorption loss of these metals is larger than that of Ag. Thus, thethickness of the Ag layer 34 is preferably equal to or greater than thepenetration depth of light, i.e., 10 nm, so that all part of the lightis reflected by the Ag layer 34 so as not to reach the protectorelectrode 50. In the case where a metal which causes a relatively smalllight absorption loss is used for the protector electrode 50, theprotector electrode 50 also has the effects of a reflection film.Therefore, the thickness of the Ag layer 34 may be less than 10 nm.

The protector electrode 50 may entirely or partially cover the electrode30. Since the protector electrode 50 is made of a metal, the electrode30 and a lead wire (not shown) can be conductively coupled together bybonding the lead wire onto the protector electrode 50 even when theprotector electrode 50 entirely covers the electrode 30. When theresistance of the metal that constitutes the protector electrode 50 islarge, the protector electrode 50 is preferably provided with an openingthrough which a lead wire can be directly bonded onto the Ag layer 34 ofthe electrode 30.

Alternatively, as shown in FIG. 9( b), a protector layer 51 made of adielectric (e.g., SiO₂, SiN) may be provided to protect the electrode30. If the electrode 30 is entirely covered with the protector layer 51,the electrode 30 could not be conductively coupled to an externalelement. Thus, it is necessary to provide an opening 52 in the protectorlayer 51 and directly bond a lead wire (not shown) onto the Ag layer 34of the electrode 30. The dielectric, such as SiO₂ or SiN, has a lowrefractive index. As such, when the protector layer 51 is formed of sucha material, the reflectance of the protector layer 51 can be furtherincreased.

By forming the protector electrode 50 shown in FIG. 9(a) or theprotector layer 51 shown in FIG. 9( b), Ag that is susceptible tomigration can be prevented from being diffused. Further, by protectingthe surface of the Ag layer 34, the Ag layer 34 is less likely to comeinto contact with sulfur or oxygen in the ambient air. Therefore,sulfuration and oxidation of the Ag layer 34 can be prevented. Notethat, among the components of the nitride-based semiconductorlight-emitting device 100 shown in FIG. 3( a), those other than theAl_(d)Ga_(e)N layer 26, the Mg layer 32, and the Ag layer 34 are notshown in FIGS. 9( a) and 9(b).

Note that metal wires (Au, AuSn, or the like) for interconnection may beformed on the above-described protector electrode 50 or protector layer51.

While the present invention has been described with respect to preferredembodiments thereof, it is to be understood that these are notlimitative examples. As a matter of course, various modifications arepossible within the scope of the invention.

Even though its structure is essentially different from the preferredembodiment of the present invention, related structures are alsodisclosed in Patent Documents 1 and 2. However, those Patent Documents 1and 2 do not mention at all that the crystallographic plane of theirgallium nitride-based semiconductor layer is an m-plane but justdisclose a technique for forming an electrode on a c-plane galliumnitride-based semiconductor layer. Specifically, Patent Document 1discloses a structure in which a thin film metal layer is formed on ap-type GaN layer, and thereafter an Ag alloy layer is formed on themetal layer. Disclosed examples of the metal used for the thin filmmetal layer are only Pt, Co, Ni, and Pd. These are metals of a largework function. Patent Document 1 seems to use these metals based on sucha common knowledge in the art that using a metal of a large workfunction for the p-electrode is preferred. The inventor of the presentapplication confirmed that, in the case of an m-plane GaN p-electrode,as described above, when simply using a metal of a large work function(Pd, Ni, Pt, or the like), the electrode of such a metal and the m-planeGaN do not form an ohmic contact. Patent Document 2 discloses electrodestructures composed of Ag, an Ag—Ni alloy, an Ag—Pd alloy, an Ag—Rhalloy, and an Ag—Pt alloy. In the electrode structures of PatentDocument 2, an alloy of a metal of a large work function and Ag isformed. This concept is also based on the common knowledge in the art.

The light-emitting device of the present invention described above couldbe used as it is as a light source. However, if the light-emittingdevice of the present invention is combined with a resin including aphosphor that produces wavelength conversion, for example, the device ofthe present invention can be used effectively as a light source with anexpanded operating wavelength range (such as a white light source).

FIG. 10 is a schematic representation illustrating an example of such awhite light source. The light source shown in FIG. 10 includes alight-emitting device 100 with the structure shown in FIG. 3( a) and aresin layer 200 in which particles of a phosphor such as YAG (YttriumAluminum Garnet) are dispersed to change the wavelength of the lightradiated from the light-emitting device 100 into a longer one. Thelight-emitting device 100 is mounted on a supporting member 220 on whicha wiring pattern has been formed. And on the supporting member 220, areflective member 240 is arranged so as to surround the light-emittingdevice 100. The resin layer 200 has been formed so as to cover thelight-emitting device 100.

In the light-emitting device 100 shown in FIG. 10, the p-electrode 30,which is an Mg/Ag electrode, is closer to the supporting member 220 thanthe semiconductor multilayer structure 20 is. Light produced in theactive layer 24 of the semiconductor multilayer structure 20 is radiallyemitted from the active layer 24. Part of the light emitted from theactive layer 24 which has passed through a light emission surface 10 aand part of the light which has been reflected by the reflective member240 are transmitted through a resin layer 200 to be emitted outside thelight-emitting device 100. In this process, part of the light isconverted to light of longer wavelengths by phosphors included in theresin layer 200. On the other hand, another part of the light emittedfrom the active layer 240 which travels toward the electrode 30 isreflected by the Ag layer of the electrode 30. Since Ag has a highreflectance for light, large part of the light that is incident on theAg layer is reflected by the Ag layer without being absorbed. The lightreflected by the Ag layer propagates through the semiconductormultilayer structure 20 and the resin layer 200 and goes out of thedevice. As a result, the amount of light emitted from the white lightsource increases. Thus, in the present embodiment, the external quantumyield can be improved.

Note that the contact structure of the present invention provides theabove-described excellent effects when the p-type semiconductor regionthat is in contact with the Mg layer is formed of a GaN-basedsemiconductor, specifically an Al_(x)In_(y)Ga_(z)N semiconductor(x+y+z=1, x≧0, y≧0, z≧0). As a matter of course, such an effect ofreducing the contact resistance can also be obtained in light-emittingdevices other than LEDs (e.g., semiconductor lasers) and devices otherthan the light-emitting devices (e.g., transistors and photodetectors).By using an Mg/Ag electrode, the contact resistance for m-plane GaN canbe greatly reduced even when the effect of reflection of light by the Aglayer is not utilized. Note that the actual m-plane does not always haveto be a plane that is exactly parallel to an m-plane but may be slightlytilted from the m-plane by 0±1 degree.

INDUSTRIAL APPLICABILITY

According to the present invention, the contact resistance between them-plane surface of the p-type semiconductor region and the p-electrodecan be reduced, and the light absorption loss in the p-electrode can bereduced. Thus, it can be preferably used for light-emitting diodes(LED).

REFERENCE SIGNS LIST

10 substrate (GaN-based substrate)

10 a light emission surface

12 surface of substrate (m-plane)

20 semiconductor multilayer structure

22 Al_(u)Ga_(v)In_(w)N layer

24 active layer

26 Al_(d)Ga_(e)N layer

30 p-electrode

30A, 30B, 30C p-electrode

32 Mg layer

34 Ag layer

40 n-electrode

42 recess

50 protector electrode

51 protector layer

52 opening

61A, 61C Mg—Ag alloy layer

100 nitride-based semiconductor light-emitting device

200 resin layer in which wavelength-converting phosphors are dispersed

220 supporting member

240 reflective member

1. A nitride-based semiconductor device, comprising: a nitride-basedsemiconductor multilayer structure including a p-type semiconductorregion, a surface of the p-type semiconductor region being an m-plane;and an electrode that is arranged on the p-type semiconductor region,wherein the p-type semiconductor region is made of anAl_(x)In_(y)Ga_(z)N semiconductor (where x+y+z=1, x≧0, y≧0, and z≧0),and the electrode includes an Mg layer which is in contact with thesurface of the p-type semiconductor region and an Ag layer which isprovided on the Mg layer.
 2. The nitride-based semiconductor device ofclaim 1, wherein the Ag layer is covered with a protector electrodewhich is made of a metal different from Ag.
 3. The nitride-basedsemiconductor device of claim 1, wherein the Ag layer is covered with aprotector layer which is made of a dielectric.
 4. The nitride-basedsemiconductor device of claim 1, wherein the semiconductor multilayerstructure includes an active layer which includes an Al_(a)In_(b)Ga_(c)Nlayer (where a+b+c=1, a≧0, b≧0 and c≧0), the active layer beingconfigured to emit light.
 5. The nitride-based semiconductor device ofclaim 1, wherein the p-type semiconductor region is a p-type contactlayer.
 6. The nitride-based semiconductor device of claim 1, wherein athickness of the Mg layer is equal to or smaller than a thickness of theAg layer.
 7. The nitride-based semiconductor device of claim 1 wherein,in the Mg layer, a concentration of N is lower than a concentration ofGa.
 8. The nitride-based semiconductor device of claim 1, furthercomprising a semiconductor substrate that supports the semiconductormultilayer structure.
 9. The nitride-based semiconductor device of claim1, wherein the p-type semiconductor region is made of GaN.
 10. Thenitride-based semiconductor device of claim 1, wherein the Mg layer andthe Ag layer are at least partially made of an alloy.
 11. A lightsource, comprising: a nitride-based semiconductor light-emitting device;and a wavelength converter including a phosphor that converts awavelength of light emitted from the nitride-based semiconductorlight-emitting device, wherein the nitride-based semiconductorlight-emitting device includes a nitride-based semiconductor multilayerstructure including a p-type semiconductor region, a surface of thep-type semiconductor region being an m-plane, and an electrode that isarranged on the p-type semiconductor region, the p-type semiconductorregion is made of an Al_(x)In_(y)Ga_(z)N semiconductor (where x+y+z=1,x≧0, y≧0, and z≧0), and the electrode includes an Mg layer which is incontact with the surface of the p-type semiconductor region and an Aglayer which is provided on the Mg layer.
 12. The light source of claim11, wherein the p-type semiconductor region is made of GaN.
 13. Thelight source of claim 11, wherein the Mg layer and the Ag layer are atleast partially made of an alloy.
 14. A method for fabricating anitride-based semiconductor device, comprising the steps of: (a)providing a substrate; (b) forming on the substrate a nitride-basedsemiconductor multilayer structure including a p-type semiconductorregion, a surface of the p-type semiconductor region being an m-plane;and (c) forming an electrode on the surface of the p-type semiconductorregion of the semiconductor multilayer structure, wherein step (c)includes forming an Mg layer on the surface of the p-type semiconductorregion, and forming an Ag layer on the Mg layer.
 15. The method of claim14, wherein step (c) further includes performing a heat treatment on theMg layer.
 16. The method of claim 15, wherein the heat treatment isperformed at a temperature of 500° C. to 700° C.
 17. The method of claim16, wherein the heat treatment is performed at a temperature of 550° C.to 600° C.
 18. The method of claim 14, wherein the step of forming theMg layer includes irradiating Mg with pulses of an electron beam suchthat Mg is deposited onto the surface of the p-type semiconductorregion.
 19. The method of claim 14, further comprising removing thesubstrate after step (b).
 20. The method of claim 14, wherein the p-typesemiconductor region is made of GaN.
 21. The method of claim 15, whereinthe Mg layer and the Ag layer are at least partially alloyed.
 22. Anitride-based semiconductor device, comprising: a nitride-basedsemiconductor multilayer structure including a p-type semiconductorregion, a surface of the p-type semiconductor region being an m-plane;and an electrode that is arranged on the p-type semiconductor region,wherein the p-type semiconductor region is made of anAl_(x)In_(y)Ga_(z)N semiconductor (where x+y+z=1, x≧0, y≧0, and z≧0),and the electrode includes an Mg island provided on the surface of thep-type semiconductor region and an Ag layer provided on the Mg island.23. A nitride-based semiconductor device, comprising: a nitride-basedsemiconductor multilayer structure including a p-type semiconductorregion, a surface of the p-type semiconductor region being an m-plane;and an electrode that is arranged on the p-type semiconductor region,wherein the p-type semiconductor region is made of anAl_(x)In_(y)Ga_(z)N semiconductor (where x+y+z=1, x≧0, y≧0, and z≧0),and the electrode includes an Mg layer which is in contact with thesurface of the p-type semiconductor region and an Ag layer which isprovided on the Mg layer, and the Mg layer is made of an Mg—Ag alloy.24. A nitride-based semiconductor device, comprising: a nitride-basedsemiconductor multilayer structure including a p-type semiconductorregion, a surface of the p-type semiconductor region being an m-plane;and an electrode that is arranged on the p-type semiconductor region,wherein the p-type semiconductor region is made of anAl_(x)In_(y)Ga_(z)N semiconductor (where x+y+z=1, x≧0, y≧0, and z≧0),the electrode is composed only of an alloy layer which is in contactwith the surface of the p-type semiconductor region, and the alloy layeris made of Mg and Ag.
 25. The nitride-based semiconductor device ofclaim 24, wherein the alloy layer is formed by forming an Mg layer so asto be in contact with the surface of the p-type semiconductor region andan Ag layer on the Mg layer, and thereafter performing a heat treatment.26. The nitride-based semiconductor device of claim 24, wherein thealloy layer is formed by depositing a mixture or compound of Mg and Agonto the surface of the p-type semiconductor region by means ofevaporation, and thereafter performing a heat treatment.